Inconsistent readings

Two weeks ago, in preparation for spring start-up, I reached under my mesh cover, and took a sample using last year's K-2005 test kit.
Values were
FC-0
TC-0
pH-7.2
TA-0
CYA-0
CH-20

I was expecting TA, CYA, and CH to be about 25% lower than last years closing due to melted snow. So I didn't believe the TA, CYA, of CH readings. So I measured three times re-read the measuring instructions and got the same results

First guess was that the reagants have gone bad, and I'll have to pick up some more. Didn't get around to it, but when when I was in the hardware store I picked up some test strips to confirm if my tests were off. The test strips provided similiar results.

Today, I removed the cover, connected the pump and started cleaning up the water. Still hadn't picked up the reagants so I thought I'd have to go to pool store to get sample measured. But first I tried my own measuremnts using last years test kit.

Cl-0
pH-7.2
TA-100
CH-250
CYA-40
Test strips measurement was reasonably close to same values.
So the issue is not my test kit reagants:confused: .


Question #1
Any idea why the difference between measurements 2 weeks ago and today?

I have two guesses, but neither seem plausible to me.

My first guess is tests provide accurate results above a specific water temperature. First reading performed at water temperature around 45F, today water temperature was 55F.

My second guess is that the pump/water circulation had an affect. Is the CYA or TA low at the water surface after a long winter. If so, vacuuming would have circulated the water around to be more uniform?


Question #2
Do I need to replace all the reagants every year or just some?